Radiometers are used to measure thermal radiation or brightness temperatures emitted from a segment of a remote object. The segment is commonly referred to as a scene and may be a portion of the earth's surface. Like most sophisticated instrumentation, radiometers require periodic calibration to insure accurate measurements. In practice, at least two known calibration temperatures, which bound the brightness temperatures of the scene, are used to calibrate a radiometer receiver. The lowest and highest calibration temperatures are referred to as cold and hot thermal radiation temperatures, respectively.
Radiometers are generally ground-based, airborne or satellite-based systems that measure brightness temperatures in the range of 10.degree. K-300.degree. K. The ground-based system may utilize closed cycle refrigeration such as a sterling cycle cooler with liquid nitrogen or liquid helium to generate cold thermal radiation temperatures "Tc". The closed cycle refrigeration systems are not considered practical for the satellite-based systems.
Referring to FIGS. 1-3, there are illustrated three traditional satellite-based systems for measuring the brightness temperature "Ta" emitted from a portion of the earth's surface and received by an antenna 36. The brightness temperature "Ta" is then transmitted through an antenna feed 32 on an antenna-earth scene line 12 to a radiometer receiver 16 of the radiometer system 150. Currently, satellite-based systems use calibration techniques that are either externally-based (FIGS. 1 and 2) or internally-based (FIG. 3).
Referring to FIG. 1, there is illustrated an externally-based calibration technique known as the sky horn approach. The sky horn approach utilizes a radiometer system 150 which includes a first RF switch 10 that connects either the antenna-earth scene line 12 or a calibration line 14 to the radiometer receiver 16. In the calibration line 14 a second RF switch 18 alternately switches between a sky horn 20 and an internal warm load 22. The sky horn 20 outputs the cold space thermal radiation temperature "Tc," approximately 2.7.degree. K, and the internal warm load 22 generates a hot thermal radiation temperature "Tw," approximately 300.degree. K. A precision thermistor 24 in thermal contact with the warm load 22 outputs an electrical hot thermal radiation temperature "Td" that is equivalent to the hot thermal radiation temperature "Tw". The electrical hot thermal radiation temperature "Td" is utilized in the calibration of the radiometer receiver 16.
The sky horn approach is a complex and expensive way to calibrate the radiometer receiver 16. The main problem is that the antenna-earth scene line 12 and calibration line 14 are separate lines, thereby requiring precise knowledge of the RF losses, mismatch losses and physical temperatures of each line to accurately calibrate the radiometer receiver 16. Also, the use of the sky horn 20 adds to the complexity of the calibration, because of possible interference of the sky horn pattern by a spacecraft or contamination caused by the earth or sun.
Referring to FIG. 2, there is illustrated another externally-based calibration technique for satellite-based systems using an antenna scanner 26. The antenna scanner 26 is a mechanical mechanism employed during a calibration mode to alternately couple a reflector plate 28 or an absorption target 30 to respectively feed a cold thermal radiation temperature "Tc" or a hot thermal radiation temperature "Tw" to the antenna feed 32. The antenna feed 32 is connected to the radiometer receiver 16. During an antenna mode when the brightness temperature "Ta" is measured, the antenna scanner 26 connects the antenna-earth scene line 12 to the radiometer receiver 16. The antenna scanner 26 does have an advantage over the sky horn approach in that only one RF path is utilized. However, the antenna scanner 26 is complex, bulky and adds significant size and weight to the radiometer system 150.
Referring to FIG. 3, there is illustrated an internally-based calibration technique that may be used in a satellite-based system. The internal approach is very similar to the sky horn approach discussed previously and illustrated in FIG. 1. However, the internal technique may utilize a thermo-electric cooler 34 to generate a cold thermal radiation temperature "Tc" of approximately 270.degree. K, instead of the sky horn 20 used in the sky horn approach. However, the hot and cold thermal radiation temperatures "Tc" and "Tw" used in the internal approach may only be 30.degree. K apart. The 300.degree. K difference between the cold and hot thermal radiation temperatures "Tc" and "Tw" does not cover the full range of earth brightness temperatures which are approximately 100.degree. K to 300.degree. K, therefore, measurement accuracy of the radiometer receiver 16 will likely degrade below the cold thermal radiation temperature "Tc."
Accordingly, there is a need for a calibration noise source to provide cold and hot thermal radiation temperatures for calibrating a radiometer. There is also a need to provide a noise source that may be manufactured using microwave integrated circuit (MIC) or monolithic microwave integrated circuit (MMIC) technologies. These and other needs are satisfied by the calibration noise source of the present invention.